snowmon,
Welcome to the forum. The PID would be your
Porter
Indicating
Device. It helps you find your beer, which is especially important while you are smoking...

Okay, the real answer is a PID is a type of temperature control system that maintains your setpoint temperature tighter than a normal on/off thermostat. Useful if you are smoking sausage (tight temperature control) or want to create the illusion that you are tending to the smoker when you are not (but having an adult beverage anyway).
A number of members here have built and installed them on their OBS, and everyone here can help you out if you want to move in that direction.
I have the Original BS in stainless steel, and have been very happy with it. I built my own PID after a year, but it really isn't necessary for most smoking. I do sausage, and wanted a bit more control without hanging around the smoker.
Arcs_n_Sparks